Balance shafts are commonly used to reduce or cancel shaking forces and/or vibrations which result from residual imbalances inherent in the design architecture of machinery with rotating parts or mechanisms, such as motors. These balance shafts are sometimes called "counterbalance" shafts.
Balance shafts are particularly valuable when operator or passenger comfort and freedom from noise and vibration-related fatigue or distraction are desired, as in the case of motor vehicles such as automobiles, motorcycles, and the like. It is also advantageous to minimize vibration from the standpoint of equipment reliability. Where vibrations are reduced, the size, mass and/or complexity of the mounting structures can often also be reliably reduced, thus potentially reducing cost.
With multicylinder motor vehicle engines, the inline four-cylinder engines and 90-degree V-6 engine configurations are favored in automotive use today due to their space efficiency and cost. Both of these engine architectures benefit from balance shafts, although for different reasons and vibratory characteristics, and thus requiring distinctly different balance shaft arrangements.
Balance shafts for inline four-cylinder engines typically are paired to rotate in opposite directions at twice the engine speed. The two balance shafts are timed to cancel each other's lateral shaking forces while opposing the vertical secondary shaking forces that are typical with this type of engine. Each shaft produces a single, or "static," rotating unbalance force, which taken together with its mating shaft's rotating unbalance force, produces a resultant vertical shaking force which most effectively is located centrally among the bank of cylinders. These static unbalance type shafts are shown, for example, in U.S. Pat. No. 4,819,505.
Other engines, such as 90-degree V-6 engines (i.e., six-cylinder engine with two banks of three cylinders spaced 90-degrees apart), produce resultant imbalance forces in the form of a crankshaft-speed rotating couple. These engines benefit from a single balance shaft with two balance "weights", or masses, on opposite sides of its axis of rotation, but spaced apart axially so as to have a dynamic imbalance providing a rotating couple. The couple produced by the balance shaft is designed to oppose or cancel that of the engine when the shaft is rotated at crankshaft speed and in the opposite direction to the crankshaft. The location of this "rotating couple"-type shaft relative to the engine is not critical so long as its axis of rotation parallels that of the crankshaft, since the output of the balance shaft is a pure couple or torque on the crankcase.
Balance shafts of both types frequently incorporate an elongated support member, or shaft, which provides a structural connection between the balance weights, in the case of rotating couple-type shafts, or between the centrally located balance weight(s) and a driving member, in the case of the static unbalance-type shaft. The elongated support member is typically subjected to both torsion and bending loads, and thus must be substantial enough to fulfill structural requirements. Since the mass of the elongated support member is largely "dead weight" and has little, if any, contribution to unbalance, its mass can be reduced in applications where overall mass is a factor in product cost and/or operating efficiency. These elongated support members or shafts typically have a circular cross-section. This circular section represents a structurally inefficient distribution of material that causes the components and their support structures to be more massive and often more costly than necessary.
The room or space for placement of balance shafts in the engine is typically small or limited. Balance shafts usually are constrained to operate within specified radii, whether to clear mating parts or to enable installation. Thus, efficient material usage typically motivates a balance weight cross-sectional shape that is, except for elongated support member intersection areas, "circular segment" in shape, i.e. the area between a radius and a chord. The radius of such a shape represents the clearance boundary beyond which the balance shaft cannot extend without risk of unwanted contact. The chord represents a locus of constant contribution to unbalance within the section, placing elements of mass equidistant from the axis of rotation, with regard to the ability of the mass element to generate centrifugal force in a particular direction, i.e., when viewed from a direction normal to the desired direction of unbalance force.
Typically, the "circular segment" shape of the balance weights are constant along their lengths. This enables easy calculation of their unbalance value from a design standpoint. However, this shape also results in inefficient distribution of material in the case of shafts with balance weights which create a rotating couple, or dynamic imbalance, thus causing components and their support structures to be more massive and thus also often more costly than necessary.
Space constraints sometimes preclude the placement, within the inline four-cylinder type engine and in conjunction with appropriate structural support, of balance weights in a manner that results in the resultant vertical shaking force being located centrally among the bank of cylinders as desired. In this situation, an unwanted pitching couple is created as a result of the axial distance between the engine's vertical shaking force and the balance shafts' resultant vertical shaking force, unless additional balance weights can be added to create rotating couple, or dynamic, unbalance within each shaft that will act to cancel this pitching couple. Such dynamic balance, when added to a static unbalance-type shaft can be seen to effectively relocate the plane of static unbalance to the new axial location where the sum of the moments of unbalance, or dynamic unbalance, within the shaft itself is zero. Any such combination of static and dynamic unbalance within a shaft can thus be characterized by an amount of pure static unbalance at an effective location or plane hereafter referred to as its "Effective Plane of Static Unbalance", or "EPSUB", about which the sum of moments of unbalance is zero.
The ideal application of balance shafts to inline four cylinder engines will locate the shafts' EPSUB at the axial center of the four cylinders, such that no pitching couple is created by an offset between the engine's shaking force and the balance shafts' shaking force, or in other words the sum of shaking force moments about the engine's axial center is zero. Where space constraints prevent this ideal full cancellation, the resulting residual shaking force may be located optimally by similar EPSUB methodology so as to most appropriately distribute the residual shaking force among engine mounts using appropriate noise, vibration and harshness minimization criteria.
Manufacturing cost consideration often force design compromises between ideal bearing configurations and ideal balance weight configurations. For example, it is common to use a larger than optimum (for friction losses, heat generation, etc.) bearing journal diameter in conjunction with a balance weight clearance boundary radius that is smaller than optimum (for unbalance creation without undue material usage) to enable axial installation (or "end loading") of the balance weight through the bearing bore, rather than incur the manufacturing complexity and cost associated with the split housing type bearings required to place an ideal configuration bearing in the midst of two larger radius balance weights that are symmetrically arrayed about the engine's center bulkhead.
The common method for providing for bearing journal diameter(s) smaller than balance weight radius without requiring split housing type bearings, namely fastening weights to a shaft after inserting the shaft through its bearing(s), is also complex, and thus also costly to manufacture, as well as being heavier than necessary.
There exists, therefore, potential for improvement in reducing manufacturing cost and solving space constraint problems, while managing the issues of drive system noise, bearing reliability, bearing drag, and overall weight in a manner that maximizes product value to the customer in the use of static unbalance balance shafts.